Four Fermions Productions at a γγ Collider

Using the recently proposed ALPHA algorithm (and the resulting code) I compute the rate (at tree level) for the process γγ → ν̄ee ud̄. The bulk of the contribution is due to W pair production and decay. However a non negligible (∼ 10%) contribution comes from other channels, mainly the production and decay of a W and a collinear charged fermion. Requiring that the reconstructed invariant ud̄ mass lies in the intervals MW ± 5 GeV and MW ± 20 GeV one obtains a rate which is lower, by 25 % and 4 % respectively, than the rate obtained in the narrow width approximation, thus demonstrating the relevance of the finite W width.